Contact protection arrangement



April 29, 1952 w. B. ELLWOOD 2,

CONTACT PROTECTION ARRANGEMENT Filed Aug. 16, 1950 lNl/E/VTUP W B.ELLWOOD Patented Apr. 29, 1952 Walter B.'Ellwood, NewYork,N. Y.,assignor to Bell Telephone Laboratories;Incorporated, New York,- N 'Y.,-a corporation of New York Application August 16, 1950, .SerialNo.179,817

(CL 175-4294) I v "8-Claims. 1 This invention relates to electricalmake-andbreakcontacts, andv more particularly to circuit arrangementsfor "protecting said contactsfrom deleteriouserosion caused byelectrical arcing and/or energy dissipationduring contact operations.

Contactsofthe'type which make and break energized electricalcircuits-develop. anumber of faults in service. For example, therepeatedmalb ing and breaking of a circuit with current flowingtherethrough often results in' irregular. arcing attended withtransferof contact metal from one cooperating 1 contact to the other, withconsequent .pitting of one contact surface and building up ofprojections -on the other. Also, the continued transfer of material fromone-contactto another often brings about a. localization of the contactarea witha resultantincrease in current density at such-restrictedareaof contact. Thislocalization condition rendersmore pronounced thedeteriorating-action cf the contactarc'ing. The transfer ofmaterialunderthese circumstances sometimes becomes so great that a sufficientamount-thereof-is built up on one of the makeand-breakcontactstopreventthebreaking of the circuit by iseparation of thecontacts.

In addition, arcing often forms thinfilms of oxides brother compounds oncontact faces thereby increasing thecontact resistance which in turntendsto aggravatethearc creating.con

-dit-ions until finally the contacts burn away or will otherwise nolonger pass current.

In order to alleviate-the eifects of arcing there exist in the prior artseveral circuit arrangements for-minimizing contact erosion.Various-resistorcapacitornetworks have been used; however, in

additionto being space consuming and difficult to install in existingplant, such arrangements haveusually proved to be ineffective on boththe .make and-break contactoperations. One such arrangement, which hasbeenwidelyused, whereina series resistor-capacitor subcombinationshunted" a contact -tobe protected proved to be very effective onthe-break contact operation but tended to be harmful on the make contactoperation. 'For complete contact protection this circuit arrangementwaspreferably employed with additional components which were to minimizecontact arcing on themake contact operation.

.As the amplitude of the vtransientsoccurring atcontacts openingandnosing circuits are inf'luencecltoagreat extent by both the. inductanceand the distributed capacitance to ground of the conductors connected vto the contacts, and as alarge. part of the-energy in these. transientswiring connected thereto. 'ness of "such choke coils was demonstrated inheated by copper.

-is-atfrequencies-about 500,000 cycles per second, it is obvious thatmodification of the high frequency or characteristic impedance ofthe-conductors connectinga contact to its associated circut "will affectthe arcing created and theenergy dissipated at the contacts duringmake-andbreak operations.

Ithas long been knownin the art that arcing occurring during contactoperations can be minimized or-eliminated by'connecting an inductorwhich has a high impedance at contact are transient frequencies inseries with the contact energizing circuit at a point close to said con-:tact. *In'early experiments high frequency choke coils" were used. toeliminate arcing. For example, in 1918 it was found that the radiofrequency interference radiated by a make-and-break spark ignitionsystem-on a submarine chaser could be greatlyreduced by inserting aniron core-single layerchoke coil between the spark points and the In1935 the usefulreducing erosion oftelephone relay contacts.

In 1938 a choke coil was suggested which comprised a permalloy platedwire wound on a permalloy shell of a relay structure and in thediscussions of thisdevice, the usefulness of permalloy plated oriron-wire as conductors in lieu of the usual copper was debated. It wasbelieved that the self conductance of permalloy plated wire or solidiron wire was sufficient to prevent arcing without the inclusion of alumped inductor close to the contacts. Oscillographic observationsshowed little difference between'the voltage transients of contactsconnected to their load by iron or permalloy-plated wire as com-.pare'd'to copper wire butrevealed that the current surges were'muchweaker'than in the case "of contacts connected bycopper wire. Life testsindicated'that differences in surge characteristics gave correspondingdifferences in erosion and it'was conclusively shown that the'erosionwas distinctly less on the contacts connected by iron andpermalloyplated'wire than on those con- During these tests it wasnoticed that the contacts connected by copper wire could easily bedistinguished from those con- "iiected by iron or permalloy plated wireas the "arc was distinctly brighter with the former.

Attempts were made to eliminate the difficulties and ex ense encounteredin using the afore- ,-mentioned iron and/or permalloy plated wire incontact circuitzarrangements. Iron wire introduced an ,objectionablyhigh direct-current resistancein many of the relay circuits connectedtherewith and also produced small iron particles due to filing at thetime of making connection therewith which lodged in contact air-gaps.Whereas, although permalloy coated copper had a satisfactory resistancevalue, the plating of copper with permalloy was expensive and also thepermalloy coating was difficult to make soldered connections to.Furthermore, a length of several feet of iron or permalloy plated wirewas required to effectively limit contact arcing. To overcome thesedifficulties the usual connecting copper conductors were passedthroughpermalloy beads or pieces of permalloy tubing adjacent themake-and-break contacts so that they provided a lumped series inductancenear said contacts. However, it was found that the eddy currentsgenerated at the high contact arcing transient frequencies created amagnetic skin effect on the surface of the beads or tubing. As aconsequence, the magnetic lines of flux in the beads created by thetransients tended to concentrate on the surface of the bead or thetubing and the inner portion of the bead or tubing was ineffectual forthe purpose of adding to the series inductive effect necessary tominimize arcing or energy dissipation at the contacts. Therefore, beadsor tubing of ordinary magnetic materials of a practical or convenientsize are not effective as contact protection due to their small value ofinductance.

Accordingly, it is an object and feature of this invention to minimizecontact erosion by employing one or more bead type inductors havingsufficient inductance at the transient frequencies present in contactarcs to minimize or eliminate contact arcing.

Another object and feature of this invention is a bead-type inductorwhich is small in size,.economical in cost, and which presentssuflicient inductance and effective alternating-current resistance in atransient make-and-break contact circuit to minimize or eliminate arcingon both the make-and-break contact operations without increasing thedirect-current resistance of said circuit.

To fulfill the objects of this invention a recently discovered magneticmaterial called ferrite in the form of a bead or the like surrounds'aconductor connecting to a contact of a relay or switch at or near thepoint where the conductor connects to the contact. This arrangementperforms the function of imparting to the surrounded portion ofconductor a high impedance at high frequencies so as to slow down anddissipate the discharge of the distributed conductor capacitance on amake contact operation, and during a break contact operation the beadinserts a dissipative resistance component to the contact connectingconductor 0 that a large'part of the stored energy of the conductorwhich causes arcing is dissipated in the loading bead instead of thecontact.

The use of ferrite beads for contact protection is advantageous becausethis material possesses the propertie of high permeability and highelectrical resistivity which provides a high effective inductancewithout lamination at frequencies in excess of a megacycle. A solid slugof the usual magnetic materials has little effective inductance at thesefrequencies because of the shielding or skin effect caused by inducededdy currents; however, the electrical resistivity of ferrites, ferritesbeing ceramic in nature, prevents the generation of high amplitude eddycurrents at high frequencies by presenting a high resistance path to theflow of these currents and at the same time maintaining a permeabilityof about 50 to 2500 depending upon the ferrite used at a flux density of10 gausses.

A ferrite has the general. formula MFezOr Where M represents a bivalentmetal. When this bivalent metal is iron, nickel, manganese, magnesiurn,cadmium or zinc, a cubic lattice structure is formed. With the exceptionof cadmium and zinc ferrites, ferrites of these metals areferromagnetic. The resistivity values range from 1O ohm-centimeter to 10ohm-centimeter compared to a value of 55x10" ohm-centimeter for 4-79molybdenum permalloy, for example. Only one of them, ferrous ferrite(Fell 6204), or magnetite, has had any appreciable use as a magneticcore material in the prior art. This is a well-known ferrite whichoccurs in nature. In its natural and synthetic form, it has been appliedcommercially in radio frequency cores for a number of years.

It is characteristic of the cubic ferrites discussed above that they canbe made to form solid solutions with each other in all proportions. Inthe recent developments in this field, certain of these solid solutionshave been found to exhibit strong ferromagnetic properties, withpermeabilities many times higher and hysteresis losses much lower thanthose of the constituent ferrites. This is brought about through properchoice and proportions of components and control of heat treatment toproduce the conditions necessary for high permeability and lowhysteresis loss; namely, low crystal anstropy and low magneto-striction.These characteristics are obtained in part through the use of anon-magnetic zinc ferrite component to reduce the Curie point; i. e.,the temperature at which the material loses its ferromagnetism. In thisway the elevated portion of the initial permeability temperature curvewhich occurs just below the Curie point can be brought within a usefultemperature range.

Although the conditions for highest permeability and lower hysteresisloss do not correspond to conditions for highest resistivity, theresistivity of the high permeability ferrite is still of the order ofohm-centimeters or higher than that of metals by a factor of 10 a Theprocess most used for producing ferrites consist of mixing the variousfinely powdered oxides in proper proportions, pressing them into thedesired shape and heat treating at approximately 1200 C. in anappropriate atmosphere. Since a linear shrinkage of around 15 per centoccurs during heat treatment, uniform density throughout the unfiredparts is necessary in'order to prevent the development of mechanicalimperfections during firing. Thi tends to restrict the forming of partsfrom dry or semidry powders to simple shapes, such as rings, discs,bars, cylinders and shallow cups. It is possible that other pressing orextruding processes can be de veloped for these in more complicatedform. However, for the purpose of making lumped inductors for contactprotection simple geometrical shapes are quite adequate.

Because of the high resistivity of ferrites, conventional eddy currentlosses are usually low so that hysteresis losses and high frequencyresidual losses become controlling. Residual loss is not fullyunderstood but in one recent explanation it is attributed to anelectronic resonance. It is a function of frequency, the loss increasingmuch more rapidly above a certain critical frequency. This rapidincrease in loss is accompanied also by a decrease in permeability. For

essence purposes of analysis the critical frequency has been-taken asthat at which the permeability drops to super cent of the direct-currentor-low frequency permeability. The low-frequency permeabilityandcritical frequency bear' -a'n inverse relationship. Therefore, for anapplication over a given frequency range, a ferrite material having anappropriate low frequency'permeability should be a chosen. For example,manganese-zinc ferrite wane permeabilityof 1200 is useful up to about '3'megacycles' while nickel-zinc ferrite with a permeability of 50-hasusable properties up to 4'0 mega'cycles. The

ferrite compounds which have'been'foundto be best suited for contactprotection are the manganese-zinc .series, the nickel-zinc series,copper-zinc series andmagnesium-zinc-series. For further informationconcerning ferrites, reference is herein made to publications by J. L.

Snoek, Non-metallic Materials for High -Frequencies, Philips TechnicalReview, December 1946, volume 8, No. 12, pages 353 to 384;New-Developments in Magnetic Materials, 1947, Elsevier PublishingCompany.

In order that the invention may be clearly understood and readilycarried into effect-it will now be fully described with reference to theaccompanying drawing, in which:

Fig.1 shows a ferrite bead having a cylindrical shape with a holetherethrough for'pas'sing a :single straight wire through said bead;

Fig. 2 shows an alternate arrangement in which a ferrite bead is used inconjunction with a single turn coil of wire so that a greater inductancevalue may be obtained for improvedcontact protection;

Fig. 3 shows an embodiment wherein ferrite beads protect switch contactsfrom arcs generated by the distributed capacitance of the connectingconductors during operation of said switch;

Fig. 4 shows how ferrite beads areapplied to switch contacts whosecircuit connection includes a twisted or cable pair of conductors; and

Fig. 5 shows ferrite bead protection directly incorporated into aswitch.

The drawing of Fig. 1 shows a cylindricalshaped ferrite bead with a holetherethrough which can be used for contact rotection by stringing saidbead on a conductor connected to the contact to be protectedtataposition preferably as close as possible to the contact. As the airsurrounding a particular'conductor hasa' permeability less than that ofthe ferrite material,

'it is'obvi-ous that the center hole should have'the .smallest'diameterC which will allow the connecting conductor to pass therethrough. Asferrite is essentially a ceramic this hole may be made during the methodsteps preceding the firing of the ceramic constituents. The bead can bemade into many other geometrical shapes; however, simple geometricalshapes are easier to form than complex shapes. It is believed that thecylindrical bead shape represents the best form of ferrite for contactprotection. vThematerial is black, hard and brittle, resembling aceramic in physical properties.

The inductance in henries'of a single conductorprovided with acylindrical bead is-expre'ssed by the equation EHBiiBi where Arepresents the diameter of the cylinder in centimeters, B represents thelength of the lit cylinder in centimeters,- C the diameter 'Of the Iliole passing therethrough in centimeters-and the permeability" of theparticularferrite used.

In Fig. 2a sin le-turn of wire I, whih is pref erably'insulated, ispassed through the holeof a ferrite head 2. Such an arrangementincreasesthe inductance imparted to a conductor passed therethroug-h by-an amountwhich varies directly as the square of the-number ofloops used,provide'd'that the beadis not otherwise magneticallysaturated.'Incertain contact' applicati'ons where space is at a premium it isdesirableto increase the series inductance by having a plurality ofturns on the bead in lieu of: using more than "a fixednu'mber' of beads.'In many applications the cost of additionalturns may exceed the costof'an equivalent number of simple beadson one "straight conductor.

'The'property'of these beads which is of particular importance in themake contact-operation-is th'e'combin'ation of high magnetic per-"me'a'bility and high electrical "resistivity which permits a higheffective inductance to'be obcurrerit resistance component which'variesfrom approiiimately 0 to 5'0 ohms per bead'for 'afr'edu'eney-range of Cito'3 megacycles'with a bead having a diameter A of approximately 03inch,and alength'B of approximately 0.5 inch. The resistance componentisreflected into the'c'onductor connected 'tothe contact and effectivelydissipates a portion of the electrical energy stored in the distributedcapacitance and inductance of the conductors connected thereto. Thisreflected resistance component also minimizes Contact arcing "3n, themake contact operation but to a lesser extent as the inductive component'predominates the resistive component in value. v

In Fig. 3 three ferrite beads are shown me circuit arrangement forprotecting a set of switch contacts,- which, for the purpose ofillustration, are enclosed in a dry reed switch vessel, said contactscontrolling a relay winding energizing circult. Ferrite beads 5 arestrung on a conductor 1 which is preferably insulated. The beads shouldbe placed as close as possible to contacts 4 so a minimum of unprotectedwiring will be to the contacts. When'a potential is applied .toterminals 3 the magnetic electrodes of the switch approach one anotheruntil the contacts 4 are made. During the -make contact operation theelectrical energy storedin the'distributed components of the circuitwill attempt to discharge through the switch thereby creating-an are atcontacts-4 if :sufiicient series impedance doesnotpreventsuchanoccurrence. The resistive and inductive components ofthe ferrite beads 5 "delay and dissipate the transient energythereby-preventing arcing at the contacts during the make contactoperation. When the steady state condition is reached, battery 9supplies energizing current to relay winding 8. During would otherwisebe dissipated in the contact with corresponding contact arcing.

Experimental tests wherein make-and-break contacts of a particular typewere connected to a battery supply over a 12-foot cable revealed alengthening of the life of said contacts of 100 to 350 per cent whenfour small ferrite beads were employed adjacent each of said contacts.

The use of ferrite beads as contact protection is especiallyadvantageous whenever a plurality of conductors are multipled to aparticular set of contacts as the value of the distributed circuitcomponents which cause troublesome arcing in creases. For example, if aplurality of conductors were connected to multiple strap 6 the increasedcapacitance to ground would aggravate the arcing at contacts 4 as thenumber of conductors connected thereto. is increased.

In Fig. 4 ferrite bead protection is incorporated in a circuitarrangement for energizing a relay winding wherein part of the circuitconnection includes a cable l4 connected to terminals [3 and relaywinding l5 so that battery 16 will energize said relay winding whencontacts H are closed by the application of a potential to terminals l0.Ferrite beads [2 function in the same manner as do the heads 5 of Fig.3. However, with a cable arrangement or twisted pair arrangementit isnecessary that only a single conductor pass through the ferrite beads asis shown in Fig. 4. If both conductors of cable 14 passed through eachof the beads the inductance of the beads would be very small due to thecanceling magnetic fields of the conductors.

In Fig. 5 a single ferrite bead is shown cooperating with an enclosedcontact of a dry reed-type switch. Bead l6 forms a collar for stationaryelectrode I1 within glass switch envelope 2|. Seals l9 and 20 fix theposition of the bead with respect to electrode H as well as providesupport for said electrode. Bead I6 functions in the same manner as theleads 5 and E2 of Figs. 3 and 4, respectively.

Such an arrangement wherein the ferrite bead is positioned closely tothe make-and-break contact electrodes l1 and i8 improves the efficiencyof the contact protection in that the impedance reflected into thecontact circuit is much closer to the arcing contact faces than ispossible in the usual case.

It is to be understood that the above-described arrangements areillustrative of the principles of the invention. Numerous otherarrangements may be devised by those skilled in the art withoutdeparting from the scope of the invention.

What is claimed is:

1. In an electrical circuit including a pair of contacts movable withrespect to one another for opening and closing said circuit to the flowof electrical current therethrough, a contact arc minimizing arrangementcomprising a conductor connected to said contacts, a mass of magneticferrite closely surrounding said conductor at a point immediatelyadjacent said pair of contacts whereby said ferrite mass imparts areflected impedance into said conductor at the contact are transientfrequencies so as to dissipate and otherwise slow down the flow ofelectrical energy to said contact during said contact make-and-breakoperations.

2. A make-and-break contact protection arrangement, as defined in claim1, wherein said mass is a manganese-zinc ferrite.

3. A make-and-break contact protection arrangement, as defined in claim1, wherein said mass is a nickel-zinc ferrite.

4. A make-and-break contact protection arrangement, as defined in claim1, wherein said mass is a copper-zinc ferrite.

5. A make-and-break contact protection arrangement, as defined in claim1, wherein said mass is a magnesium-zinc ferrite.

6. In an electrical circuit including a pair of contacts movable withrespect to one another for opening and closing said circuit to the fiowof electrical current therethrough, a contact arc minimizing arrangementcomprising an electrical conductor for carrying current to and from saidpair of contacts, a mass or magnetic ferrite material having anelectrical resistivity in the range of 10- ohm-centimeters to 10ohm-centimeters at room temperature and a magnetic permeability in therangeof 50 to 2500 at a magnetic flux density of 10 gausses at amagnetizing current frequency within the range of 0.5 to 3.0 megacycles,and said ferrite mass being magnetically coupled to said conductor by aclose physical positioning of said mass to the conductor portionimmediately adjacent said contacts.

7. In an electrical circuit including a pair of make-and-break contactsfor controlling the flow of current in said circuit, a contact areminimizing arrangement comprising a conductor connected to said contactsand a head of magnetic ferrite with a hole therein through which saidconductor is passed and said bead physically positioned adjacent saidcontacts whereby a change of magnetic flux density in said bead inducesan electromotive force in said conductor during make-contact operations.

8. In an electrical circuit including a sealed switch having a pair ofconducting electrodes with make-and-break contacts thereon, a contactare minimizing arrangement comprising a mass of magnetic ferritemagnetically coupled to one of said conducting electrodes by physicallypositioning said mass closely to an enclosed conducting electrode ofsaid switch whereby said mass imparts a reflected impedance into saidconducting electrode during contact operations so as to minimize contactarcing.

WALTER B. ELLWOOD.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,032,658 Clement July 16, 19121,773,938 Bishop Aug. 23, 1930 1,840,089 Gilbert Jan. 5, 1932 2,228,798Wassermann Jan. 14, 1941 2,298,468 Curtis Oct. 13, 1942 2,313,809 CurtisMar. 16, 1943 2,375,609 Zuhlke May 8, 1945 2,452,529 Snoek Oct. 26, 1948

